Recent unique findings from our laboratory suggest that the togavirus, eastern equine encephalomyelitis (EEE) virus, causes significant cytopathology in the midgut of its mosquito vector, Culiseta melanura. This observation challenges previous beliefs regarding the benign nature of mosquito infection with arboviruses, and suggests potential mechanisms which may 1) facilitate rapid virus dissemination within the mosquito and transmission to vertebrates, and 2) act to alter the virus gene pool during vector infection. Using light and electron microscopy, we propose to further evaluate togavirus-induced vector pathology in the EEE virus -Cs. melanura system, as well as in another medically important alphavirus, Venezuelan equine encephalomyelitis virus, in its enzootic mosquito vector, Culex taeniopus. These studies will establish the effect of togavirus infection on the biological fitness of the mosquito, including blood digestion, reproduction, host seeking and longevity, and define the relationship between midgut pathology and virus dissemination in the vector. Using a molecular approach, we will examine the genetic composition of arbovirus populations over time during various stages of rapid and persistent mosquito infection. Our proposed research, which couples classical entomological techniques with electron microscopy and molecular biology, will reveal mechanisms of arbovirus-vector coevolution, which will lead to better understanding of the epidemiology of togaviral diseases.